It is known that this type of paper must particular have specific mechanical properties, namely high breaking strain and folding resistance, and which are homogeneity and texture which are appropriate for the production of a watermark. Moreover, the paper must satisfy very stringent criteria particularly in respect of its dimensional stability, its optical brightness and its cleaness.
The traditional preparation of a paper pulp intended for the manufacture of a banknote paper is carried out in a plurality of distinct steps.
Firstly, the material, generally comprising cotton fibers originating either from comber wastes or in the form of textile wastes, or comprising other textile fibers such as ramie or linen, is initially subjected to coarse chopping and is placed in a bin. The fibers and/or aggregations of fibers are then compacted by means of a pestle in a rotating chest, as a result of which, on emerging from this apparatus the stock is in the form of annular rings.
The annular rings, also called "cakes" are then introduced into a digester where the boiling and bleaching operations are carried out jointly with the aid of an aqueous solution of caustic soda and hydrogen peroxide. This operation is carried out under a pressure in the vicinity of atmospheric pressure and at a temperature of the order of 90.degree. to 95.degree. C. While the bleached cakes are still in the digester, they are washed with clean water in order to remove the boiling and bleaching reagents.
The compacting operation, carried out with the aid of the pestle, before introduction into the digester, essentially serves to increase the density of the stock so as to make optimum use of the volume of the said digester.
The bleached and washed cakes, after extraction from the digester, are chopped by means of a special machine of the milling cutter type.
The stock is then treated in breaker engines or with the aid of another system suited for this fibrous stock. This operation, carried out in water, furthermore makes it possible to separate the textile fibers and to cut them uniformly into 3 to 4 mm lengths.
The fibers thus obtained, still accompanied by water, are introduced into a stuff chest and constitute the first stuff or half-stuff.
The first stuff then passes to a disc-type refiner where the fibers are refined. This operation modifies the physical structure of the fibers and imparts to the paper the physical and mechanical properties required for currency use.
The whole stuff then enters a mixing chest in which is added pulp originating from the recycling of reeler or trimmer shavings and brakes. This mixing chest then feeds a stuff chest situated at the head of the paper machine as such.
It can be seen, therefore, that the traditional manufacture of a banknote paper corresponds to a discontinuous method, and entails numerous handling operations to load and unload the items of apparatus, more particularly for the stamping of the cakes and the boiling and milling of the bleached cakes.